US 8127544 B2
This invention provides a compact, fuel-efficient internal combustion engine that can be used to provide rotating shaft output power to a wide variety of mobile and stationary applications. It is based on a two-stroke free-piston gas generator that implements the homogeneous charge compression ignition (HCCI) combustion principle for essentially constant-volume combustion, and it employs a variable piston stroke to maintain a high level of efficiency across a wide range of loads and speeds. A rotary device, which may be of either an aerodynamic or positive displacement type, converts the energetic gas stream to power at a rotating shaft.
1. An engine system comprising:
a) at least one combustor producing energetic hot gas, said combustor comprising:
a cylindrical casing closed at each end by a cylinder head,
a piston assembly oscillating in the cylindrical casing, said piston assembly comprising two pistons fixedly attached to each other by a rigid connecting rod,
a divider element centrally disposed within the cylindrical casing and penetrated by the rigid connecting rod through a central bore, thereby forming two separate cylinders within the cylindrical casing, with a piston operating in each,
an air inlet communicating with each cylinder for admitting and controlling the flow of combustion air,
a fuel inlet communicating with each cylinder to introduce fuel into and mix fuel with said combustion air;
at least one combustion chamber formed in each cylinder between each cylinder head and each piston head,
at least one spark plug being mounted in the at least one combustion chamber of the at least one combustor;
a source of electric power for generating an electrical spark to initiate combustion during initial start-up until the oscillation of the piston assembly compresses the air/fuel mixture in the at least one combustion chamber to auto-ignition temperature, and
an outlet in each cylinder for discharging and controlling the flow of energetic hot gas produced by the auto-ignition of the air/fuel mixture in the at least one combustion chamber;
b) a gas-driven motor to convert energy of the energetic hot gas produced in the at least one combustor to mechanical energy delivered to an external load via a rotating shaft, wherein said gas-driven motor is powered entirely by said energetic hot gas and further comprises:
a rotating element being driven by the energetic hot gas from the at least one combustor based on at least one of aerodynamic flow, pressure, and positive displacement forces,
the rotating shaft connected to the rotating element to transmit a mechanical output power to the external load,
an inlet for the energetic hot gas from the at least one combustor to be supplied to the gas-driven motor, and
an outlet to discharge the energetic hot gas to the ambient atmosphere after expansion by the gas-driven motor;
c) at least one duct connecting the outlet of each cylinder of the at least one combustor to the gas-driven motor to supply the energetic hot gas from the at least one combustor to the gas-driven motor; and
d) a rotary pre-compressor to deliver combustion air to the at least one combustor
wherein said rotary pre-compressor is driven by a short shaft turned by an auxiliary gas-driven motor, said auxiliary gas-driven motor driven by a portion of the energetic hot gas produced by the at least one combustor.
2. The engine system of
the air inlet further comprising an intake port disposed in a wall of the cylinder and axially adjacent to one face of the divider element;
an intake chamber bounded by the underside of the piston and one face of the divider element, and communicating with the intake port;
at least one lower transfer port communicating with the intake chamber;
at least one upper transfer port communicating with the combustion chamber; and
at least one transfer passage connecting the at least one lower transfer port to the at least one upper transfer port;
whereby the oscillating motion of the piston forces at least one of combustion air and air/fuel mixture to pass through said at least one lower transfer port to said at least one upper transfer port, scavenging the combustion chamber once the energetic hot gas has been discharged through the outlet.
3. The engine system of
4. The engine system of
a source of compressed air; and
at least one three-way valve communicating with the compressed air source and with each intake chamber;
whereby compressed air is alternately introduced into and vented from each intake chamber so as to move the piston assembly into a favorable position for initiating combustion in each combustion chamber.
5. The engine system of
the divider element having a diameter that is smaller than the bore of the cylindrical casing;
two restraining elements disposed within the cylindrical casing near each face of the divider element; and
two compressible rings, one disposed between each restraining element and each face of the divider element;
whereby the divider element radially moves to the degree allowed by the flexible rings.
6. The engine system of
the rigid connecting rod such that said rigid connecting rod is hollow;
a radial passage in the interior of the divider element running from its circumference to its central bore;
a bushing introduced into the central bore of the divider element, wherein said bushing is split into two halves, with a toroidal space between the inner ends of each half;
a port in the wall of the rigid connecting rod;
a supply line for conveying a lubricating substance through the radial passage into the toroidal space between the inner ends of each half of the bushing, thence to the interior of the rigid connecting rod, and thence to each piston; and
one or more ports in each piston to allow the lubricating substance to flow between each piston and the walls of each cylinder.
7. The engine system of
at least one permanent magnet or electromagnet fixedly attached to the rigid connecting rod; and
at least one electrical coil fixedly attached to the divider element and oriented such that the rigid connecting rod and the at least one permanent magnet or electromagnet pass through said at least one electrical coil;
whereby the changing magnetic field generated by the motion of the at least one permanent magnet or electromagnet through the at least one electrical coil induces an electrical pulse to indicate piston position.
8. The engine system of
a source of pressurized fuel; and
an electronic fuel injector disposed in the combustion chamber;
whereby the electrical pulse provides a timed signal that the electronic fuel injector uses to introduce fuel into, and mix fuel with, the combustion air before the air/fuel mixture is raised to ignition temperature.
9. The engine system of
whereby the electronic control unit uses the electrical pulse to determine both frequency and phase of the harmonic motion of the piston assembly and to provide dynamic operating parameters to various engine systems and instruments.
10. The engine system of
This invention pertains to thermodynamically efficient internal combustion engines, particularly U.S. Patent Classifications 123/46R (free piston engines), 123/46A (free piston engines with two chambers and one piston), 123/46B (free piston engines-phasing means between two or more units) and 60/595 (power plants in which a free piston device supplies motive fluid to a motor). In addition to the field of free-piston engines, the invention is relevant to internal combustion engines employing homogeneous charge compression ignition (HCCI). It is also relevant to the efficient operation of internal combustion gas turbine engines. In one embodiment employing compressed air starting, the invention pertains to U.S. Patent Classification 60/596 (power plants using a free piston device with a pressure fluid starting means).
Two-stroke free-piston engines used in combination with a rotary, gas-driven motor (often referred to as “compound free piston-gas turbine engines,” of which the present invention is a subtype) are known and have been proposed as a means of improving fuel efficiency, reducing cost, and adding multifuel capability to conventional gas turbine engines. Such compound engines have also been proposed as a means of improving fuel efficiency, increasing power density, improving low-speed torque, reducing cost and complexity, and adding multifuel capability to conventional crank-piston internal combustion engines. Most early designs employed a free-piston combustor (gas generator) of the “two-piston opposed” type to generate pressurized gas, and used the pressurized gas to rotate an axial power turbine, as instanced by A. F. Underwood, “The GMR 4-4 ‘Hyprex’ Engine: A Concept of the Free-Piston Engine for Automotive Use,” SAE Transactions 65 (1957): 377-391. Known prototypes demonstrated an ability to run on a wide variety of liquid fuels, had very low vibration, and exhibited excellent low-speed torque. However, they proved to have fundamental structural weaknesses and were very bulky and heavy. Fuel efficiency was reported to be roughly equal to or slightly better than that of conventional crank piston engines.
A proposed improvement to the “two-piston opposed” model for compound free piston-gas turbine engines might be described as a “single-piston” or “single piston assembly” model, as instanced by U.S. Pat. No. 1,785,643, U.S. Pat. No. 2,963,008, and U.S. Pat. No. 4,205,528. These and similar designs incorporate a free-piston combustor (gas generator) in which a single, double-ended piston, or alternatively a single assembly consisting of two pistons fixedly attached to each other via a rigid connecting rod, oscillates back and forth in either a single cylinder or two coaxial cylinders, with combustion occurring at alternate cylinder ends (heads). The pressurized gas thus produced is used to rotate a power turbine to do useful work, while the piston motion itself is used solely to produce compression in the cylinder end opposite each combustion event. Such designs claim greatly increased operating frequency of the free-piston combustor (gas generator), as well as superior structural strength, greater simplicity, and dramatically reduced weight and size, relative to the “two-piston opposed” model.
Within the past twenty years, free piston engines of the single piston/single piston assembly type have been proposed as a means of obtaining homogeneous charge compression ignition (HCCI), which offers advantages over conventional combustion models in terms of near-instantaneous burn rate (hence nearly ideal constant volume combustion), the enabling of high compression ratios, the ability to run unthrottled at very lean mixtures, multifuel capability, and reduced particulate and NOx emissions. Free-piston engines, and more particularly free-piston engines of the single piston/single piston assembly type, are a natural fit for HCCI combustion: because such engines have inherently variable stroke, a compressing force may be applied indefinitely to a homogeneous charge of any temperature and equivalence ratio until the charge auto-ignites, and, because piston motion is unconstrained, the exact moment of combustion need not be controlled. This is in marked contrast to existing crank-piston HCCI engines, in which engine temperature, temperature of the intake charge, and equivalence ratio must be carefully monitored and controlled to ensure that spontaneous ignition occurs at or near top dead center of piston motion. Crank-piston HCCI engines additionally face the potential for rods and cranks to be damaged by the high pressure peaks characteristic of HCCI combustion: because of this, they are typically run at very lean mixtures and constant low loads. Free piston engines, by contrast, have no conventional rods or cranks, and can thus theoretically be operated across a wide power range at mixtures up to and including the stoichiometric ratio without damage to engine components.
While several recent designs utilize HCCI combustion in a two-stroke free-piston engine of the single piston/single piston assembly type (e.g., U.S. Pat. No. 6,199,519 and U.S. Pat. No. 6,959,672), none employs the engine as a combustor (gas generator) where the energetic output gas is used to drive a rotary motor. The first example above is configured as an electrical linear alternator, for instance, while the second assumes a configuration as either an electrical linear alternator or a hydraulic or pneumatic pump. All other known configurations of two-stroke, HCCI free-piston engines of the prior art incorporate one of these three power extraction methods. These configurations fail to take full advantage of the free-piston model for HCCI operation: because they attempt to extract useful work from the piston motion, piston speed and momentum are reduced, and the engine's ability to generate a compressing force sufficient for auto-ignition of the charge is thereby rendered problematic. In practice, most such engines require complex sensing and control mechanisms to balance compression force with power extraction forces, and they have thus been confined to constant-load operation. Also, because extracting work from piston motion reduces the maximum attainable operating frequency of the engine, such engines have exhibited poor power density. A superior solution is suggested by utilizing a power turbine or rotary gas-driven motor for power extraction in two-stroke free-piston HCCI engines of this type, but there is no known instance of such a design in the prior art.
The prior art does contain at least one instance of a two-stroke, HCCI, free-piston combustor (gas generator) in isolation, namely that described by J. Horton (“Amazing New Lightweight Turbine,” Mechanix Illustrated [February, 1969]: 66-68; 134-136). Because it is configured as a pure jet with no attempt made to extract work from the piston motion, the Horton engine enjoys several advantages over a similar engine configured as a linear alternator or hydraulic or pneumatic pump, including simplicity (no sensing or control mechanisms are required, and the engine has three moving parts), extremely high operating frequency, and very light weight. However, since the Horton engine is a pure jet, it has a limited range of applications (for instance, it is unsuitable for a land vehicle operated in traffic). Additionally, the Horton engine is reported to suffer from extreme noise, as well as from typical two-stroke disadvantages of needing to add lubricant to the fuel and of short-circuiting raw fuel out the exhaust. Finally, the Horton engine employs an engine geometry that requires very precise machining and that is nonetheless vulnerable to leakage and binding. All of these challenges are addressed by the currently preferred embodiment of the present invention.
Special mention should be made of two additional patents: U.S. Pat. No. 1,785,643 (referenced earlier) and U.S. Pat. No. 7,258,086. U.S. Pat. No. 1,785,643 (Noack et al.) describes a single-piston assembly, two-stroke, compound free piston-gas turbine engine in which an integral, reciprocating linear electric motor/generator is coupled to a rotary generator in order to obtain favorably-phased oscillation rates of the piston assemblies in multiple free-piston combustors (gas generators). While the Noack engine did not utilize HCCI combustion, the use of a linear motor/generator to obtain these favorably-phased oscillation rates is similar to that put forward in an alternative embodiment of the present invention, which will be described later in greater detail. U.S. Pat. No. 7,258,086 (Fitzgerald) describes a four-cylinder, four-stroke, HCCI, compound free piston-gas turbine engine as one alternative embodiment. While this stated embodiment shares several operating principles with the currently preferred embodiment of the present invention, numerous distinctions arise from the unique architecture required to support four-stroke vs. two-stroke operation. Relative to the compound free piston-gas turbine embodiment claimed in U.S. Pat. No. 7,258,086, the currently preferred embodiment of the present invention: 1) uses half the number of pistons and cylinders to obtain the same number of power strokes; 2) eliminates a moving piston linkage; 3) utilizes simple ports rather than a positive valving system; 4) employs an under-piston chamber to provide scavenging pressure rather than employing a separate intake stroke; 5) employs closed-cylinder fuel injection in preference to open; 6) employs a novel, closed-circuit lubrication system, and 7) provides for the optional substitution of a positive-displacement gas-driven motor and/or centrifugal turbine in place of the “power turbine” named in the Fitzgerald embodiment. A four-stroke architecture is one valid means of addressing common two-stroke problems of a narrow power band, poor fuel efficiency, and high emissions; however, the currently preferred embodiment of the present invention utilizes alternative means to address these problems and eschews a four-stroke architecture in favor of the higher operating frequency, improved power density, decreased complexity and cost, and reduced friction inherent in its two-stroke configuration.
The present invention is based on the type of two-stroke, single-piston assembly, compound free piston-gas turbine engine design described in the prior art. That is, it consists of at least a free-piston combustor (gas generator) to compress, combust, and partially expand a charge, and a rotary device that uses the energetic, hot gas thereby obtained to turn an output power shaft. In contrast to the prior art regarding such compound engines, the present invention employs homogeneous charge compression ignition (HCCI) as the combustion model: this is done to take advantage of the superiority of HCCI over other combustion models in terms of thermodynamic efficiency, multifuel capability, and reduced emissions. The currently preferred embodiment includes a rotary supercharger or turbocharger to provide initial compression of input air to the free-piston engine; an additional embodiment allows for the substitution of a positive-displacement gas-driven motor and/or centrifugal turbine in place of the axial- or impulse-type power turbines of the prior art. In order to start the engine, compressed air means and a conventional ignition system are employed; an alternative embodiment puts forth an integral linear electric motor/alternator to start the engine and, in the event that multiple free-piston combustors (gas generators) are used, to obtain favorably-phased oscillation rates of the piston assemblies of such combustors (gas generators).
One way of looking at the configuration of the present invention is to consider it similar to a conventional gas turbine engine (turboshaft) in which the higher-pressure stages of the compressor, the combustor can(s), and the first stages of the compressor turbine have been replaced by the free-piston combustor (gas generator). By using the positive displacement device of free pistons to quickly and efficiently attain compression rather than using the inefficient dynamic device of multiple rotating compressor stages, and by employing intermittent, closed, near-ideal constant-volume combustion in preference to continuous, open-ended, constant-pressure combustion, the present invention operates at a much higher thermodynamic efficiency than a conventional gas turbine engine.
An alternative way of looking at the present invention is to consider it similar to a supercharged or turbocharged two-stroke crank-piston engine with the crank and connecting rods removed. This allows for easy accommodation of HCCI combustion, which is more thermodynamically efficient than either compression-ignition direct-inject (“diesel”) or homogeneous-charge spark-ignition (“petrol”) combustion modes. Freeing the pistons to operate with the sole constraints of fluid and inertial forces places the structure of the engine under much less stress than crank-piston configurations; it also decouples the frequency of combustion from the rotational speed of the output shaft, allowing for higher operating frequencies, improved power density, rapid delivery of full power from idle, and the development of extremely high torque at low shaft output speeds.
The operation of the free-piston combustor (gas generator) portion of the currently preferred embodiment greatly reduces the complications of the prior art. It has one primary moving part: a single assembly consisting of two pistons fixedly attached to each other via a rigid connecting rod, oscillating in a single cylindrical “casing” closed at each end and divided into two functionally separate cylinders by a central, axially fixed, disk-shaped divider element. The only other moving parts are two passive reed valves controlling the intake air flow and two in-cylinder fuel injectors. (Both of these last elements are optional, but are utilized by the currently preferred embodiment.) Simple intake ports, upper and lower transfer ports, and outlet ports are introduced into the cylinder walls.
In the currently preferred embodiment, combustion occurs in an alternating fashion at either cylinder end, between the top of each piston head and its respective cylinder head. For purposes of illustration, the sequence of events is described beginning with the piston/connecting rod assembly closest to the cylinder head on one side of the device: for instance, the left side. In this position, the charge in the left-side combustion chamber has been compressed to the point of auto-ignition. Once it ignites, the piston/connecting rod assembly begins its expansion stroke toward the right. On the right side, fresh intake air is drawn past the right-side intake port and reed valve into the chamber formed between the underside of the right-side piston and the central, axially fixed divider element. Meanwhile, in the right-side combustion chamber formed between the piston head and cylinder head once the piston head has closed the outlet port, the intake air from the previous stroke is compressed and fuel is injected. At the same time, the fresh intake air pulled into the left-side under-piston chamber on the previous stroke is compressed by the motion of the piston toward the central divider element. Once the left-side piston head has passed the left-side outlet port, the expanding combustion products from the left-side combustion chamber are evacuated through the outlet port toward the power turbine (or positive-displacement gas-driven motor, in an alternative embodiment). When evacuation is complete, the left-side piston uncovers the left-side upper transfer port, and pressurized fresh air from the left-side under-piston chamber is admitted through this transfer port into the left side combustion chamber, scavenging any remaining combustion products. The piston/connecting rod assembly then reaches its farthest extent on the right side, the right-side charge auto-ignites, and the cycle begins again. The operation is similar to that of a standard twin-cylinder, twin-piston engine using a two-stroke cycle, except that there is no crank, no conventional crankcase, no conventional rods connecting the pistons to the crank, and no conventional spark-ignition system.
Several advantages of the currently preferred embodiment over conventional gas turbine engines, conventional crank-piston engines, crank-piston HCCI engines, compound free piston-gas turbine engines of the prior art, and free-piston HCCI engines configured as electrical linear alternators or hydraulic or pneumatic pumps have already been enumerated in the previous section. Further advantages over these and additional engine types are detailed below.
In comparison to a conventional gas turbine engine: 1) The present invention is able to reduce manufacturing costs, since an easily machined free-piston combustor (gas generator) and conventional supercharger or turbocharger replace multiple, high-precision compressor and compressor turbine stages. 2) The present invention can be idled at much lower fuel consumption, since the variable stroke of the free-piston combustor (gas generator) enables it to attain maximum compression at idle. 3) The present invention's use of a piston-based combustor (gas generator) eliminates the possibility of compressor stall in the event of a sudden application of full load to a previously unloaded engine. 4) The present invention operates with relatively low temperatures at the inlet nozzles of the precompressor turbine and power turbine (or gas-driven motor), since peak combustion temperatures are attained at the top of the piston stroke and are greatly reduced by the time the working fluid is expanded at the end of the piston stroke and directed toward the precompressor turbine and power turbine/gas-driven motor. This feature enables noncritical materials to be used in the manufacture of precompressor turbine and power turbine/gas-driven motor, further decreasing manufacturing costs (see Underwood, p. 379).
In comparison to two- and four-stroke compression-ignition direct-inject (“diesel”) crank-piston engines and two- and four-stroke spark-ignition (“petrol”) crank-piston engines: 1) Thermodynamic efficiency is improved though the utilization of the Pescara thermodynamic cycle, which eliminates the energy losses inherent in Otto and Diesel cycles and approximates a Miller or Atkinson cycle in terms of allowing the effective expansion stroke to be longer than the compression stroke. 2) The rapid burn rate and high piston speed of the present invention improve thermodynamic efficiency by reducing the time available for the heat of combustion to transfer to cylinder walls. 3) Engine efficiency is improved via the elimination of side loads, decreased reciprocating masses, and reduced incidences of sliding friction from conventional connecting rods, crank bearings, cams, and camshafts. 4) Both weight and cost are reduced via the elimination of crankshaft, conventional connecting rods, flywheel, valves, and camshaft. 5) The reduced duration of high combustion temperatures and the reduced peak temperatures of HCCI combustion at low loads and equivalence ratios results in the drastic reduction of NOx emissions. At the same time, particulate emissions are reduced as a result of the high fuel atomization and complete burning inherent in HCCI combustion. (See U.S. Pat. No. 6,199,519, FIGS. 8; 11-16; also Energy Efficiency and Renewable Energy, Office of Transportation Technologies, “Homogeneous Charge Compression Ignition [HCCI] Technology: A Report to the U.S. Congress, April 2001” [U.S. Department of Energy, Washington, D.C., 2001], pp. 1-5.) 6) The very high compression ratios attainable in the present invention facilitate uniformly high temperatures of the compressed charge and thus enable the utilization of a wide range of fuel types, including high-viscosity/low-volatility fuels such as Bunker C, Jet-A, kerosene, diesel oil, vegetable oil, and certain types of unrefined crude oil (See Underwood, p. 378). It is anticipated that the present invention should also operate satisfactorily on gaseous fuels such as butane, CNG, LPG, and pure and impure hydrogen, as well as renewable fuels such as biodiesel, pure ethanol, and pure methanol, without engine modification (see U.S. Pat. No. 6,199,519). Finally, it is anticipated that gasoline and other hydrocarbon fuels of poor quality and/or very low octane and cetane ratings may be utilized, as knock inhibition is not required.
Relative to spark-ignited crank-piston engines specifically: 1) Thermodynamic efficiency is improved though the facilitation of higher compression ratios, since knock, or detonation, is not a limiting factor to compression. In fact, detonation is utilized by the present invention and all HCCI engines as the normal operating mode. 2) The present invention is capable of running at equivalence ratios well below what is possible in conventional spark-ignited engines. This ability to run at lean mixtures leads to better thermodynamic efficiency and allows the engine to be run unthrottled, greatly reducing pumping losses. 3) Since lean mixtures require higher initial temperatures to auto-ignite than do richer mixtures, still higher compression ratios are facilitated during lean running. 4) Finally, variable stroke ensures a consistently high effective compression ratio over all load and speed requirements: the engine increases and decreases its working displacement automatically as load and speed dictate. This is a crucial advantage where a wide range of loads and speeds is anticipated, as in an automobile or other mobile application.
Relative to two-stroke spark-ignited crank-piston (“petrol”) engines specifically: 1) The currently preferred embodiment of the present invention utilizes non-critically-timed low-pressure fuel injection into the cylinder after the outlet port closes, preventing loss of fuel through the outlet port. 2) The currently preferred embodiment utilizes a closed-circuit lubrication system in which lubricant is introduced through the hollow connecting rod and does not directly enter the under-piston chamber or mix with the fuel. This reduces undesirable emissions by reducing the amount of lubricant that is burned during combustion. (For the currently preferred embodiment, standard multiweight motor oil is contemplated as a lubricant. Additional lubricant options may include vegetable oils and solid or semi-solid lubricants, as well as unconventional lubricants such as water or gaseous elements.)
Finally, relative to four-stroke rotary (Wankel) engines, the present invention offers improved compression ratios, particularly at low loads. It has reduced incidences of sliding friction, as well as improved sealing, a faster burn rate, and an ability to run at low equivalence ratios, which the spark-ignited Wankel engine cannot accommodate.
Divider element 43 shown in
The upper transfer ports 47 in
Note that the compressed-air starting system of
As stated above, the linear electric motor depicted in
In conclusion, the combination of a two-stroke free-piston engine with a power turbine to provide rotating shaft output is not in itself new. What is new is the incorporation of the HCCI combustion model into this compound engine type. Additional novel elements include: 1) the division of the single cylinder casing into two functionally separate chambers by the use of an axially fixed, radially floating divider element; 2) the optional incorporation of non-critically-timed fuel injection into the cylinder after the outlet port closes; 3) the optional use of a closed-circuit lubrication system in the free-piston portion of the engine; 4) optional inclusion of an integral electrical linear motor for starting and possible phase control of multiple free-piston combustors (gas generators) coupled to a single output shaft; and 5) the use of a positive-displacement gas-driven motor as one way of utilizing the high-pressure gas output of the free-piston combustor (gas generator). Note that the use of a precompressor, as well as the substitution of a positive-displacement gas-driven motor for an axial or centrifugal power turbine, are all optional in the present invention. A precompressor increases the power density of the engine, but is not required. Similarly, for some applications, the positive-displacement gas-driven motor option for power extraction makes better use of the level of mass air flow inherent in the free-piston engine, but this may not be true in all applications.
Although only preferred embodiments of the present invention are specifically disclosed and described above, it will be appreciated that many modifications, variations, substitutions, and equivalents are possible in light of the above teachings and within the purview of the appended claims, without departing from the spirit and intended scope of the invention.
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